This invention relates to proximity sensors and more particularly to an indiscriminate and intermingled use of almost any type of sensors especially--although not exclusively--well adapted to control an automatic shutdown of machine tools, primarily for the protection of the human operators.
Recent Federal legislation requires machine tools to be controlled in a prescribed manner whenever there is a safety hazzard to the worker. Thus, for example, a punch press must be prevented from closing when a worker's hands are too near the moving parts. Since an automatic machine shutdown is desirable to protect the worker, it is also desirable to use the same equipment to protect the machine from damaging itself. For example, a part stuck in a die, or a raw stock which does not properly feed could ruin an expensive tool and die. Thus, it is desirable to prevent the machine from operating if either of these malfunctions should occur.
Another consideration is that no one sensor is able to serve all proximity detection needs in a simple and efficient manner. For example, if a sensor is to detect a person entering a door, a simple light beam across the door is sufficient. On the other hand, if the detector is to detect a person on a spiral staircase, the light beam is almost totally inadequate since it would have to be duplicated for nearly every stair tread. Likewise, those skilled in the art will readily perceive when still other forms of sensors are to be desired.
I have taught the use of an antenna sensor in my earlier U.S. Pat. No. 3,573,783, granted Apr. 6, 1971. That sensor functions very well and has enjoyed commercial success. However, it tends to become unduly complex under some conditions, as when a light beam sensor, for example, is more adequate than an antenna. On the other hand, an antenna is preferable to a light beam sensor if line of sight detection is not desirable or adequate to system needs.